Martin -- I worry that we may be talking at cross-purposes, here. <xenoData> is for supplying metadata, just like the other <teiHeader> components are for supplying metadata. Said metadata might be *about* the original source document (here is the MODS record for the book that was transcribed), and it might be *about* the TEI document (here is the Dublin Core metadata for my TEI file that we can use on ingestion into XTF). In the former case, it is not about the <sourceDesc>, it's about the source. In the latter case it is about the <TEI>, but so is everything else in the <teiHeader>, that's the point of having a <teiHeader>. (Some people are arguing it may also be *about* something else, e.g. the page images associated with this TEI document.) So a precise pointing mechanism does not solve the problem "what is this metadata about". That said, just like most any other metadata in the <teiHeader>, the metadata may be applicable to only certain portions of my collection. E.g., I may have obtained my copy of _The Fellowship of the Ring_ from one library, and my copies of _The Two Towers_ and _The Return of the King_ from another. The @decls mechanism already exists to handle this. We don't need another mechanism to solve the problem of "to what portions of my TEI document is this metadata applicable". (That said, you may want to suggest we should improve the @decls mechanism -- I can already hear you saying it should be global -- but that's a different ticket. :-)
@decls is available only a small subset of elements, and not on any of the header elements. This doesn't allow you to say that a specific <xenoData> applies to <sourceDesc> and another to <profileDesc>, does it?